1. Field of the Invention
The invention relates to the general field of the silicone coating, on high-speed rolls, of various flexible supports, such as sheets of paper or of synthetic polymer (polyolefin, polyester, etc), or else of textile.
More specifically the invention concerns the coating of flexible materials with liquid compositions comprising one or more polyorganosiloxanes crosslinkable by polyaddition, by dehydrocondensation, by polycondensation, cationically or free-radically to form a protective coating or film having, in particular, release and/or water-repellency properties.
2. Description of Related Art
The flexible supports may be papers, cards, plastic films or metallic films. The applications of these silicone-coated supports are, for example: paper for food use (baking molds, wrapping), adhesive label/tape, packing and sealing material, etc.
The coating of these flexible supports with crosslinkable liquid silicones is carried out on coating devices which operate continuously and at very high speed. These devices comprise coating heads composed of a number of rolls, including in particular a press roll and a coating roll, which is fed continuously with crosslinkable liquid silicone composition, by means of a series of rolls which are associated with one another. The web of flexible support circulates at high speed between the press roll and the coating roll and is thereby coated on at least one of its faces with a silicone film which is intended to be crosslinked by crosslinking means disposed downstream of the coating head. These crosslinking means may be emitters of heat, of radiation (e.g., ultraviolet) or of electron beams, for example.
In the race for productivity, the producers of silicone release-coated flexible supports are customers for liquid silicone coating formulations which are suited to increasingly high linear running speeds of the flexible support web. The economic factor is obviously not insignificant in this search for new silicone formulations for high-speed coating.
Nevertheless, the high speeds on continuous coating machines are known to be a byword for problems of transfer of the liquid silicone film from the coating roll to the moving flexible support web. These transfer problems (“splitting”) are manifested, in particular, in the incidence of a mist or aerosol (“misting”, “fogging”) in the area around the coating head and, more particularly, at the points of contact between the rotating rolls and/or between the coating roll and the flexible support to be coated. The density of this mist or of this aerosol increases in line with the linear running speed and hence the speed of rotation of the rolls.
Consequences of this phenomenon are, first of all, a loss of consumable material, and in particular the deposition of droplets of coating liquid on the support downstream (for example, at the oven), which is seriously detrimental to the quality of the coating.
Moreover, this undesirable formation of mist has adverse consequences from the standpoints of industrial hygiene and of safety for the operatives, who are exposed to a high level of aerosol in the vicinity of the roll coating device. This aerosol may be toxic.
Furthermore, the misting gives rise to the rapid fouling of the roll coating device, causing maintenance constraints and premature wear.
To guard against the consequences of this mist, it is usual to dispose a suction withdrawal system around the coating head, allowing said mist to be captured.
Moreover, the skilled worker knows of a certain number of adjustments to the coating head in order to obviate this phenomenon. Some examples of this include:    A. lowering the speed, which is detrimental to productivity;    B. reducing the silicone deposition rate, which is detrimental to the properties of the flexible silicone support it is desired to obtain (appearance, covering, release, mechanical properties);    C. increasing the difference between the tangential speed of the coating roll and the linear speed of the paper; however, beyond a certain differential, the homogeneity of the coating layer is severely disturbed; moreover, it is possible by this means to reduce the density of the mist without eliminating it sufficiently to allow a significant increase in coating speed;    D. increasing the pressure between the coating roll and the press roll; here again, to a certain limit, and without advantageous suppression of the phenomenon of mist formation.
Another approach for controlling the formation of mist in roll coating machines involves acting on the formulation of the liquid silicone coating composition.
In accordance with this approach, it is known to reduce the number-average degree of polymerization of the polyorganosiloxanes forming the silicone coating liquid and, consequently, to reduce the viscosity of the silicone coating bath so as to limit the density of the mist.
These known techniques are subject to a serious drawback, in that they substantially modify the properties and, in particular, the release of the flexible silicone-treated support it is desired to obtain.
To illustrate this approach by way of the silicone formulation, it is possible to cite international patent application WO 2004/046248, which describes the use of star-branched silicone polymers used as an antimisting additive for coating applications on flexible supports. The process for preparing these star-branched silicone polymers comprises incompletely reacting (by hydrosilylation) a polyorganosiloxane containing reactive ≡SiH units with a long-chain olefin to give a partially substituted polyhydroorgano-siloxane, which is subsequently reacted by hydrosilylation with a vinyl silicone resin of MQ type and a long-chain diolefin. It is clear that compositions of this kind are relatively complex and therefore costly to obtain. Moreover, they still remain capable of improvement in terms of controlling misting in high-speed silicone roll coating.
European patent EP-0 716 115 describes a process for preparing a silicone composition for high-speed coating with rolls, said composition being presented as permitting a reduction in mist density. According to this process, a trimethylsilyl-terminated polydimethylmethylhydrosiloxane with a degree of polymerization of 12, and also 0.01% of a polydimethylsiloxane which is substituted by perfluoroethylbutyl and methylvinyl functions, whose end groups are dimethylvinylsiloxy groups, and whose degree of polymerization is 300, and also polypropylene glycol and, optionally, a stearyl or oleyl alcohol are employed. This leads to polydimethylsiloxanes which are functionalized with polyoxypropylene groups. These functionalized polydimethylsiloxanes are combined with other functionalized polydimethylsiloxanes, functionalized for example with hexenyl units, and are also combined with a platinum-based hydrosilylation catalyst, to form silicone coating compositions which permit a reduction in mist formation. The functionalization units may be hydrophobic residues such as stearic or oleic acid residues.
U.S. Pat. No. 4,806,391 relates to silicone-based inks and varnishes, and more specifically to a method of applying these inks/varnishes to a substrate, using a roller coating machine operating at high speed. This patent discloses, in particular, compositions comprising vinyl-terminated polydimethylsiloxanes with a viscosity at 25° C. of between 15 000 and 50 000 mPas. These liquid coating compositions further comprise a platinum-based catalyst and a rheological additive composed of silica with a high specific surface area, more particularly fumed silica.
The American U.S. Pat. No. 6,057,033 discloses silicone compositions intended for coating on flexible supports to form, after UV-induced cationic crosslinking, a release coating. In addition to the polyorganosiloxanes, these compositions comprise cellulose fibers which have an average length of between 15 and 100 μm and an average thickness of between 5 and 40 μm. The polyorganosiloxanes employed are polyorganosiloxanes which are functionalized with crosslinking groups of acryloxy or methacryloxy type, allowing UV-induced free-radical crosslinking.
The cellulose fibers incorporated into the composition make it possible to provide a solution to the technical problem, which is that of obtaining a nonbrittle crosslinked silicone release coating. The cellulose fibers are presented as producing improvements with regard to the transfer of the silicone coating film to the support, resistance to die cutting, mechanical properties (tensile resistance and tearing resistance), the anchoring of the coating to the paper, the reduction of the absorption of the coating liquid within the paper, and, incidentally, the reduction of mist formation.
On this last point, U.S. Pat. No. 6,057,033 does not provide any quantitative element for assessing the reduction in mist to which the cellulosic fibers give rise. There is good reason to think that this reduction remains completely inadequate.
Also cited, for report, is Japanese patent application JP-62 64 011, which describes a coating liquid comprising a film-forming resin and a solvent and which further comprises wax particles with a diameter of between 1 and 10 μl, the diameter of the coarsest particle being not more than 150% of the thickness of the wet coating film applied to the support. A coating liquid of this kind would allow an increase in coating speed of at least 10 to 30 m/min, by virtue a priori of a limitation on the formation of mist. The teaching of such a document is remote since it does not relate to silicone coatings.